[Boost-users] Re: Re: Re: Re: Re: Re: Re: result_of tutorial

19 Mar 2005


      Peter Dimov wrote:
...
Peter Dimov wrote:
...
Ian McCulloch wrote:
...
Peter Dimov wrote:
[...]
...
I don't get it: can't you detect whether
nested::result::type exists?  As long as the primary
template of nested::result is defined as an empty struct this should
work fine(?).
No, consider what happens when 'nested' is int (*) (), for example.
Hm, it can't be int (*) (), because result_of will not attempt to look for
a nested result, but it can be an user-defined type with no nested result,
or a member named result.
Wouldn't both those cases (no nested result, or a member named result) cause
a substitution failure?


#include <iostream>

struct foo
{
   template <typename T>
   struct result { typedef int type; };
};

struct bar
{
   template <typename T>
   struct result {};
};

struct baz
{
   void result() {}
};

struct bat
{
};

void func(...)
{
   std::cout << "func(...)\n";
}

template <typename T>
typename T::result::type
func(T const& x)
{
   std::cout << "func(T const&)\n";
   return typename T::result::type();
}

int main()
{
   func(foo());
   func(bar());
   func(baz());
   func(bat());
}

This compiles cleanly with g++ 3.3.4 and produces
func(T const&)
func(...)
func(...)
func(...)


Cheers,
Ian